In the fabrication of a variety of electronic devices with microstructures, such as semiconductor devices and liquid crystal devices, a lithography method is widely used, and, as the device structures are miniaturized, resist patterns in the lithography process are desired to be miniaturized.
Currently, in a state-of-the-art lithography technique, a fine resist pattern having a line width as small as, for example, 90 nm can be formed by the lithography method, and the formation of a further finer pattern will be needed in the future.
A first point for achieving the formation of such a fine pattern that is smaller than 90 nm, is to develop an improved exposure system and a corresponding resist. The point of the development of the improved exposure system generally includes provision of a light source, such as an F2 laser, an EUV (extreme ultraviolet light), an electron beam, or an X-ray, a soft X-ray, with a shorter wavelength and of a lens having an increased numerical aperture (NA).
However, the light source having a shorter wavelength has a problem in that it requires a new expensive exposure system, and the increase of NA has a problem in that since there is a trade-off between the resolution and the depth of focus, the increase in the resolution lowers the depth of focus.
Recently, as a lithography technique which can solve the problems, a liquid immersion lithography method has been reported (cf., for example, Literature 1 (J. Vac. Sci. Technol. B(1999) 17(6), p 3306-3309), Literature 2 (J. Vac. Sci. Technol. B(2001) 19(6), p 2353-2356, and Literature 3 (Proceedings of SPIE Vol. 4691 (2002), p 459-465). In this method, a resist film is exposed through a liquid refractive index medium (refractive index liquid, immersion liquid), such as pure water or a fluorocarbon inert liquid, having a predetermined thickness, with the liquid refractive index medium lying at least on the resist film between a lens and the resist film on a substrate. In this method, the space of the path of exposure light, which has conventionally been filled with an inert gas, such as air or nitrogen gas, is replaced by a liquid having a larger refractive index (n), for example, pure water, with the result that even though a light source having a wavelength for the exposure conventionally used is employed, high resolution can be achieved without lowering the depth of focus like the case where a light source having a shorter wavelength or a lens having a higher NA is used.
By employing the liquid immersion lithography, a resist pattern having a higher resolution and an excellent depth of focus can be formed at a low cost using a lens mounted on the existing exposure system, so that the liquid immersion lithography has attracted a considerable attention.
However, in the liquid immersion lithography process, the resist film is directly in contact with the refractive index liquid (immersion liquid) during the exposure, and hence the resist film is vulnerable to invasion by the liquid. Therefore, it is necessary to check whether the resist composition conventionally used can be applied as is to the liquid immersion lithography process.
The resist composition currently commonly used are established compositions that were obtained as a result of examination of a wide variety of candidate resins in respect of the transparency to the exposure light, which is the most essential property to the resist compositions. The inventors of the present invention have made experiments and studies with a view to obtaining from the currently proposed resist compositions a resist composition having properties suitable for the liquid immersion lithography as such or after slightly adjusting the formulation of the resist composition. As a result, it has been found that a promising resist composition from a practical point of view is present. On the other hand, it has been confirmed that there are a number of resist compositions which cannot achieve satisfactory resolution of pattern in the liquid immersion lithography due to a change in their properties by the liquid, but which exhibit fine and high resolution in ordinary lithography employing the exposure through a layer of air. Such resist compositions have been established by spending considerable resources on the development, and are excellent in various resist properties, such as transparency to the exposure light, development properties, and storage stability. These resist compositions include a number of compositions which are poor only in the resistance to the immersion liquid. Some examples of the compositions, which are not suitable for the liquid immersion lithography but achieve high resolution in the lithography through a layer of air, are shown in the Comparative Examples of the present invention described later.
It has also been confirmed that, even when the resist film suitable for the liquid immersion lithography is used in the liquid immersion lithography, the quality and non-defective yield are slightly poor, as compared to those obtained in the exposure through a layer of air.
The suitability for liquid immersion lithography of the conventional resist film was evaluated based on the following analysis on the liquid immersion lithography process.
Specifically, for evaluating the performance of formation of a resist pattern by the liquid immersion lithography, it is considered to be necessary and sufficient to confirm three points, i.e., (i) the performance of the optical system in the liquid immersion lithography process, (ii) the effect of the resist film on the immersion liquid, and (iii) the change of properties of the resist film due to the immersion liquid.
With respect to the performance of the optical system in (i), as is apparent from the case where for example, a photographic sensitive plate having a surface resistance to water is immersed in water and the surface of the plate is irradiated with a pattern light, there will be no problem in principle if no light transmission loss, such as reflection, occurs at the water surface and the interface between the water and the surface of the sensitive plate. The light transmission loss in this case can be easily removed by optimizing the angle of incidence of the exposure light. Thus, it is considered that any objects of the exposure, for example, a resist film, a photographic sensitive plate, and an image screen, cause no change in the performance of the optical system, so far as they are inert to the immersion liquid, namely, they are not affected by the immersion liquid, and they do not affect the immersion liquid. Therefore, a check test for this point is not required.
The effect of the resist film on the immersion liquid in (ii) specifically indicates that the component of the resist film is dissolved in the immersion liquid to change the refractive index of the liquid. Theoretically, when the refractive index of the immersion liquid changes, the optical resolution of the pattern exposure is sure to change, and experiments are unnecessary. It is enough to simply check whether the component of the resist film immersed in a liquid is dissolved in the liquid to change the formulation or refractive index of the immersion liquid, and it is unnecessary to check the resolution by actual irradiation of a pattern light and development.
Conversely, when the resist film immersed in the liquid is irradiated with a pattern light and developed to check the resolution, it is possible to know as to whether the resolution is excellent or poor. However, it is difficult to judge whether the resolution is affected by the change of properties of the immersion liquid or the change of properties of the resist material or both.
With respect to the phenomenon in which the resolution is lowered by the change of properties of the resist film due to the immersion liquid in (iii), an evaluation test such that “the resist film after the exposure is showered with the immersion liquid and then developed, and the resultant resist pattern is examined in respect of the resolution” is satisfactory. In this evaluation method, the resist film is directly showered with the liquid, and hence the conditions for immersion are very stringent. In this point, in the test in which the resist film completely immersed in the liquid is exposed, it is difficult to judge whether the resolution is changed by the change of properties of the immersion liquid, the change of properties of the resist composition due to the immersion liquid, or both.
The phenomena (ii) and (iii) above are two sides of the same coin, and can be grasped by merely checking the change of properties of the resist film due to the liquid.
Based on the results of the analysis, the suitability for liquid immersion lithography of the resist film currently proposed was evaluated by an evaluation test such that “the resist film after the exposure is showered with the immersion liquid and then developed, and the resultant resist pattern is examined in respect of the resolution”. The suitability can also be evaluated by simulating the practical production process using a “two-beam interferometry exposure method” that includes using an interfered light caused by a prism as a pattern light for exposure and subjecting a sample immersed in a liquid to exposure.
As mentioned above, for producing a new resist film suitable for the liquid immersion lithography, a large investment of resources for the development is surely needed. On the other hand, it has been confirmed that resist compositions having qualities slightly lowered but having properties suitable for the liquid immersion lithography are obtained from the currently proposed resist compositions by using the resin compositions as such or by adjusting the formulation of the resin compositions. It has also been confirmed that there are a number of resist films which cannot achieve satisfactory resolution of pattern in the liquid immersion lithography due to the change of properties by the immersion liquid, but which exhibit fine and high resolution in ordinary lithography employing the exposure through a layer of air.